This invention relates to copolymers and, more particularly, to a triblock copolymer based on poly(glycolic acid) and absorbable surgical articles, including a flexible monofilament suture or ligature manufactured from the copolymer.
Synthetic absorbable sutures manufactured from polyglycolic acid are well known and have met with commercial success. Generally, such sutures are manufactured, sold and used as braids. However, some surgeons prefer the suturing characteristics of a monofilament suture. It has long been recognized that a need exists for an absorbable monofilament suture which exhibits the advantageous properties of strength, flexibility and absorbability possessed by polyglycolic acid braided sutures.
There have been various prior art suggestions for modifying polyglycolic acid, such as by copolymerization of glycolide monomer with other monomers, to produce a polymer possessing the requisite properties desired in a monofilament suture. For example, U.S. Pat. No. 4,243,775, Rosensaft and Webb, assigned to the assignee of the present invention, discloses a polymer material useful for forming both an absorbable braided suture and, under certain conditions, flexible monofilament suture with extended strength retention. That patent discloses the sequential addition of a cyclic ester monomer, such as a lactide, lactone, oxalate or carbonate, to glycolide monomer in the copolymerization process. Triblock copolymers with lactic acid units predominantly on both ends of a glycolide polymer chain are disclosed as are copolymers of trimethylene carbonate and glycolide and monofilament sutures made therefrom.
Other copolymers for use as bioabsorbable materials have been disclosed. U.S. Pat. No. 4,048,256, assigned to the assignee of the present invention, discloses a normally solid bioabsorbable hydrolyzable polymeric reaction product of a polyglycolic acid composition and a polyester of diglycolic acid and an unhindered glycol. Copolymers of 1(-)lactide with glycolide have also been used as suture material as disclosed in U.S. Pat. No. 3,636,956. Polyethylene oxide/polyethylene terephthalate copolymers have been disclosed as biodegradable elastomeric biomaterials in Reed, et al., "Trans. Am. Soc. Artif. Intern. Organs", 1977, page 109. The production of copolymers based on monomers formed from lactic acid or glycolic acid has been known for nonbiological purposes. U.S. Pat. No. 2,917,410 discloses the condensation of glycolic acid with a polyethylene glycol mixture to form an ester with an average molecular weight of 5105 for treating fabric for improved tear strength and abrasion resistance. The addition of aromatic orthocarbonates during the formation of a fiber-forming polyester by the reaction of a dicarboxylic acid or its functional derivative with a glycol is disclosed in U.S. Pat. No. 3,714,125.
U.S. Pat. No. 4,070,347 discloses poly(orthoester) co- and homopolymers and poly(orthocarbonate) co- and homopolymers useful for forming delivery devices with drug dispersed therein for release by controlled polymeric erosion over a prolonged period of time. Glycolide and ethylene glycol are reacted to form an oligomer which is polymerized by condensation with various monomers to yield the desired polymers. The copolymers contain only isolated oxyethylene connecting groups and the orthocarbonate units contain ring structures.
In order to produce an acceptable synthetic absorbable suture, monofilament fibers manufactured from a copolymer must meet certain requirements besides absorbability including good handling properties, adequate tensile and knot strength, avoidance of unfavorable tissue reactions, ability to be sterilized without significantly affecting desired properties and controllable uniformity in the desired properties. The acceptability of a suture is frequently determined by the Youngs modulus (a measurement of flexibility), the tensile strength and the percent elongation at the breaking point (a measure of extensibility).